A local oscillator (LO) is a device used to generate a signal which is beat against the signal of interest to mix it to a different frequency. The LO produces a signal which is injected into a mixer along with the signal from an antenna (e.g., or from a low noise amplifier (LNA) coupled to the antenna) or to the antenna (e.g., or to a power amplifier (PA) coupled to the antenna) in order to effectively produce the sum or difference of the two signals.
There are several traditionally known LO schemes. Firstly, a poly phase module coupled to an oscillator or a quadrature oscillator is used as a LO generator. However, if the LO generator under this scheme is used for a low IF transmit architecture which involves a PA, frequency of the LO generator output ends up being very close to the output frequency of the PA being transmitted where unwanted feedback from the PA to the LO generator can corrupt the LO generator output.
Another traditionally known scheme involves a frequency divider (e.g., with a factor of K) coupled to an oscillator. Since output frequency of a PA used for transmission is separated by output frequency of a LO generator coupled to the PA by the factor of K, this scheme can avoid the problem faced by the earlier traditional scheme. However, if a non-linear PA is used as the power amplifier (e.g., having more than 30 mV power output), Kth harmonic of the PA output can become very close to the frequency of the LO generator output, where RF coupling paths can lead to corruption of the LO generator output.
FIG. 1A illustrates a traditional LO generator 100 implemented to rectify the problems discussed in the earlier two LO generator schemes. In FIG. 1A, an oscillator 102 generates a 1.6 GHz signal which is forwarded to a frequency divider 104 with a dividing factor of 2. Two buffers (e.g., a buffer 106 and a buffer 108) and a synthesizer 110 functions as a phase-locked loop (PLL) circuit which responds to both frequency and phase of the input signal (e.g., from a receiver input or to a transmitter output) by automatically raising or lowering frequency of the oscillator 102 until it is matched to the reference in both the frequency and the phase.
The in-phase and quadrature components having the frequency of 800 MHz coming out of the frequency divider 104 are added to the 1.6 GHz signal using a mixer 112 and a mixer 114, respectively, to generate 2.4 GHz in-phase and quadrature signals. The signals are forwarded to receiver (RX) mixers 116 through a buffer 118 and a buffer 120 during the receive mode. Alternatively, the signals are forwarded to transmitter (TX) mixers 122 through a buffer 124 and a buffer 126.
For modulation schemes or power levels where output of the transmitter power amplifier does not disturb the oscillator 102 (e.g., the output of the PA is much less than 30 mV), the scheme illustrated in FIG. 1A consumes additional power. In addition, during the receive mode which does not involve the PA, there is an additional power overhead due to the mixer 112 and the mixer 114 used to generate the signals being forwarded to the RX mixers 116.
Furthermore, as illustrated in frequency spectrum 150 of FIG. 1B, the LO generator 100 in FIG. 1A generates spurious harmonics (e.g., 0.8 GHz, 1.6 GHz, 3.2 GHz, etc.) due to mixing of the signals at the mixer 112 and the mixer 114. As a result, the LO generator 100 is forced to implement extra components (e.g., the buffer 118, the buffer 120, the buffer 124 and the buffer 126) to filter the emission of the spurious harmonics during the transmit mode and/or ward off jammers mixing with the spurious harmonics during the receive mode.